The invention relates to a system for lifting and moving heavy loads, especially for use in the installation or removal of offshore platforms, comprising a number of lifting devices which are arranged for mounting on a floating structure which during use floats on the surface of a volume of water beside the load which has to be lifted.
Various types of systems and devices are known in the prior art for lifting heavy sections during the installation or removal of offshore platforms. Conventional methods used in this connection have usually been based on the use of offshore crane ships or heavy lift vessels. In a typical operation of this kind, a crane ship will be positioned close to a platform and lift the various sections of the platform in a predetermined sequence.
Systems or devices are also known which are designed to lift the entire upper part or deck of a platform in a single operation. Such a system, which is of the type mentioned at the beginning, is the so-called xe2x80x9cVersatrussxe2x80x9d lifting system. This system is a twin-barge lifting system which is based on two barges which are located at a suitable distance apart oil opposite sides of the structure which has to be lifted, and which are interconnected and can be pulled towards each other with great force by means of winch devices. On each barge are placed a number of lifting beams which are tilted inwards and upwards in the direction of the load, and which are brought into engagement with the load. The two barges are then pulled towards each other, thus causing the angles of inclination of the lifting beams to increase as the distance between the barges decreases, thereby causing the load to be lifted up in tile area between the barges which then forms a catamaran configuration.
This known system is based on custom-built lifting beams which therefore have to be specially made or adapted for each lifting operation. Furthermore, the system is restricted to use in relatively calm waters, without particularly large waves, since very heavy lifts of up to 20 000 tons are involved here, and thus very great forces, with the result that large waves can easily cause serious damage to or destruction of tile equipment, since the system does not possess any kind of heave compensation.
Another known device in this field is the so-called xe2x80x9coffshore shuttlexe2x80x9d. This is a U-shaped unmanned offshore vessel based on tugboats for operational assistance. During installation or removal of platform structures the vessel is ballasted so that it is submerged and surrounds the upper part of the platform or the deck, and then deballasted so that the structure is lifted. A typical xe2x80x9coffshore shuttlexe2x80x9d may be 150 m long, 80 m wide and 60 in high with a weight of around 12 000 tons. A large structure is therefore involved here. The structure has the advantage of being extremely stable in a submerged condition, in the same way as a semi-submersible structure or xe2x80x9csemi-subxe2x80x9d. However, this principle requires a corresponding depth at the application site.
In practice the hitherto known concepts, which are based on the semi-sub principle or on two interconnected vessels in a catamaran configuration, will be faced with serious problems. In the case of a semi-sub, one of the main problems is the time it takes to evacuate the ballast to a point where the semi-submerged structure makes contact with the object which has to be lifted, and the time it takes to perform the lift. The problems involved are naturally associated with the heaving motion experienced by the floating objects as a result of wave action.
As mentioned above a semi-sub will be relatively stable and relatively little affected by waves. Assuming, however, that the waves have an influence, the mass forces which are set in motion will have to be absorbed when tile floating structure encounters the object which has to be lifted. The serious consequences involved can easily be imagined if such a floating structure (12000 tons) should experience a heaving motion of 1 m and hit a platform deck on the way up from a wave trough.
The second problem which may be encountered is when there is a failure to perform a lift to a safe height within a wave period when using a semi-sub solution or a two-vessel system. The consequence thereof can be that tile load (the platform deck) is replaced on the foundation from which it was lifted, thereby causing damage or possibly losing the entire lift, or that an acceleration is experienced during installation as a result of faulty xe2x80x9ctimingxe2x80x9d.
A further problem in connection with such lifting operations is the uncertainty which reigns concerning the distribution of weights on a platform deck. In earlier times there was no adequate documentation and control of the building process, nor were the subsequent modifications to the platforms completely documented. This can lead to ignorance of the platform deck""s centre of gravity, with the result that a controlled deballasting, of the floating structures cannot be prepared in order to take this factor into account. The consequences can be a tilted lift, or in the worst case failure to perform the lift.
In view of this, it is an object of the invention to provide a system which has substantial lifting power and inherent heave compensation, where the system can control the forces transferred to the lift object during the entire lifting operation, and a lift to a safe height can be accomplished within a wave period, with the result that the system is suitable for lifting extremely heavy loads while being reliable in operation.
A second object of the invention is to provide a system of this kind which is cost-effective and module-based, thus enabling several lifting devices to be connected together as required.
In order to achieve the above-mentioned objects, a system is provided of the type mentioned in the introduction which according to the invention is characterized in that each lifting device comprises a lever arm unit with a first and a second arm projecting in opposite directions from a common mounting point, the first arm having a lifting point at its free end for engaging with the load, at least one first container which is connected to the first arm at a point near the said lifting point and which is arranged to receive and discharge a flowable medium and to be submerged in the volume of water, and at least one second container which is suspended at the flee end of the second arm, the interior of the container being connected via a pipeline device, and a device is provided for fast transfer of medium in the first container via the pipeline device to the second container.
The system according to the invention is cost-effective since it is based on the use of floating structures in the form of existing barges or other suitable vessels which can be hired. The lifting devices will be prefabricated and modular, thus enabling the system to be easily transported to the site where a lifting operation has to be performed. A typical system for lifting a platform will be comprised of two barges in a catamaran configuration, with two or more lifting devices located on each barge. It may be expedient to provide a hydraulic auxiliary system with hydraulic cylinders which are connected between respective lever arm units and the floating structure in order to ensure that interacting lever arms are lifted in parallel and uniformly, and thus enabling the lift can be carried out in a controlled manner.
By combining a floating structure with substantial load capacity with one or more partly submerged containers in the manner indicated, the advantage of both stability and lifting power is obtained, thus providing minimal movement in the water and maximum lifting power. By transferring the force in the manner indicated by means of weight transfer from the partly submerged containers at one end of the lever arm unit, a passive heave compensation is obtained and particularly a progressive heave compensation as the containers are increasingly submerged.